1. Technical Field
The present invention relates to an image forming apparatus using an electrophotographic process, and in particular, to a method for controlling a transfer bias of a transfer roller which transfers toner image from a photosensitive drum to a paper.
2. Related Art
An electrophotographic process is widely employed for an image forming apparatus such as a copier, a laser beam printer (LBP), a facsimile machine, etc. The electrophotographic process commonly consists of the successive steps of charging.fwdarw.exposing.fwdarw.developing.fwdarw.transferring.fwdarw.fixi ng. First, a photosensitive drum is uniformly charged to a predetermined potential, and the charged portion of the photosensitive drum is exposed to a light image of an original document being reproduced. This creates an electrostatic latent image corresponding to the document image on the photosensitive drum. After the electrostatic latent image is formed on the photosensitive drum, the latent image is developed by applying toner particles from a developing unit into contact with the latent image formed on the photosensitive drum. This forms a toner image of the original document on the photosensitive drum which is subsequently transferred and fixed on a recording medium. When a toner image is fixed on a recording medium, the toner image is first heated and fused onto the recording medium, and then naturally cooled so that it is fixed onto the recording medium.
Generally, as a unit for charging the surface of the photosensitive drum, a contact-type charging roller is used to produce an uniform electric field in response to application of high voltage for charging the surface of the photosensitive drum to a constant potential to attract toner particles and thereby form the latent image on the photosensitive drum. Typically, a charging unit is charged at the start of a printing operation. The surface of the photosensitive drum is charged and a developing unit is concurrently charged by way of the charging unit as the photosensitive drum rotates in a direction opposite to the rotation of the developing unit. As a result, an electrostatic latent image is formed on the photosensitive drum and the latent image is then visualized as a toner image by the developing unit. The charged area of the photosensitive drum is then exposed to a laser beam. Because of the potential difference between the exposed area and the unexposed area of the photosensitive drum, the toner particles are attracted only to the exposed area to form the toner image on the photosensitive drum. The toner image is then transferred to the recording medium. After the toner image is transferred to the recording medium, the photosensitive drum is charged back to an original voltage as the recording medium is being conveyed to a fixing unit. A common problem to contemporary charging unit, as I have observed, is that toner supplied from the developing unit often sticks on an unexposed area so that both edges of the recording medium can be contaminated.
Conventionally, there are a number of known techniques for controlling the charge and surface potential of a photosensitive drum to form an electrostatic latent image thereon as described, for example, in U.S. Pat. No. 4,839,695 for Device For Controlling Charge Area Of Photoreceptor issued to Yamamoto et al., U.S. Pat. No. 5,072,258 for Method Of Controlling Surface Potential Of Photoconductive Element issued to Harada, U.S. Pat. No. 5,164,779 for Image Forming Apparatus With Dual Voltage Supplies For Selectively Charging And Discharging An Image Bearing Member issued to Araya et al., U.S. Pat. No. 5,247,328 for Method And Apparatus For Charging A Photoconductive Surface To A Uniform Potential issued to Daunton et al., U.S. Pat. No. 5,287,149 for Image Forming Apparatus Having Image Transfer Electrode Contactable To Transfer Material issued to Hoshika, U.S. Pat. No. 5,450,180 for Image Forming Apparatus Having Constant Current And Voltage Control In The Charging And Transfer Regions issued to Ohzeki et al., U.S. Pat. No. 5,479,243 for Image Forming Apparatus And Charging Device Thereof issued to Kurokawa, U.S. Pat. No. 5,512,982 for Image Forming Apparatus With A Photosensitive Member And A Charging Device Having An Oscillatory Voltage Source issued to Takahashi et al., U.S. Pat. No. 5,517,289 for Apparatus for And Method Of Forming Image issued to Ito et al., U.S. Pat. No. 5,534,982 for Developing Apparatus issued to Sakaizawa et al., U.S. Pat. No. 5,557,375 for Contact Type Charging Device And Image Forming Apparatus Having The Same issued to Nagayasu et al., and U.S. Pat. No. 5,715,499 for Contact Charger Having Oscillating Voltage For Charging A Photosensitive Member issued to Yamazaki. While these conventional charging techniques contain their own merits, I have noted that none can efficiently minimize contamination of inversely charged toner from the photosensitive drum at front and rear ends of the paper due to the abrupt bias voltage difference. Hence, the front and rear ends of the paper may be contaminated more severely than other places of the paper.